1. Technical Field
The disclosure is related to a type of line driver, and in particular to a type of line driver which adaptively adjusts an output impedance.
2. Background
FIG. 1 is a schematic diagram of a front-end circuit of a conventional transmitter. Referring to FIG. 1, during a process of transmitting signals, a digital to analog converter 110 converts a digital signal DS1 from a digital system into an analog signal AS1. Next, a line driver 120 transmits the signal AS1 to a transmission line 130. Two types of impedance exist at the junction between the line driver 120 and the transmission line 130: one is an output impedance Ro1 of the line driver 120, and the other is an equivalent impedance Zin1 formed by a characteristic impedance of the transmission line 130 and a load resistor R1.
FIGS. 2A and 2B are each a schematic diagram showing transmission of signals. As shown in FIG. 2A, when the output impedance Ro1 and the impedance Zin1 are not equal, an output signal ST21 of the line driver 120 induces a great reflection signal SR21. At this moment, the output signal ST21 is interfered by the reflection signal SR21, so that efficiency of the transmission system is reduced. On the other hand, as shown in FIG. 2B, when the output impedance Ro1 and the impedance Zin1 are approximately equal, i.e. impedance matching, a reflection signal SR22 is far less than the output signal ST21. At this moment, the output signal ST21 is affected by the reflection signal SR22 to a limited extent, so that the efficiency of the transmission system is barely affected. In other words, impedance matching between the line driver 120 and the transmission line 130 directly affects the quality of signal transmission.
In order to enhance the stability and efficiency of a transmission system, U.S. Pat. No. 5,936,393 discloses a type of “Analog Line Driver with Adaptive Impedance Matching.” In this patent, a line driver utilizes an integrator to perform integration on a whole signal and utilizes a voltage obtained by integration to adjust a voltage on a resistor. Thus, as the voltage on the resistor changes, the output impedance of the line driver changes accordingly. However, since integration is performed on the whole signal in this patent, this technology cannot be applied to a system which has a fixed direct-current voltage. Moreover, errors during manufacturing of the resistor cause serious distortion of greater output signals, thereby affecting quality of signal transmission.
Furthermore, in 2002, the Institute of Electrical and Electronic Engineers (IEEE) published the article “An adaptive analog video line driver with impedance matching based on peak detector” in pages 273-276 of volume 4 of the journal Circuits and Systems. This publication discloses using two peak detectors to detect a difference between signal amplitudes and using a detection result to adjust a current that flows through a resistor, thereby changing an output impedance of a line driver accordingly. However, this method requires extra hardware costs for the system, and the disposed resistor causes serious distortion of greater output signals.